Referring now to FIG. 1, bulk wet etching is a commonly used process for creating a thin membrane (such as 14) or through-holes in silicon wafer 11 by using a hot alkaline etchant 13, such as KOH, TMAH (tetramethyl ammonium hydroxide), EDP (ethylene diamine pyrocatechol), etc. In this process, the wafer front side, where various devices and/or circuits 17 are located, must be isolated from any contact with the etchant. However, due to non-uniformity of etching and/or defects of the backside etching mask 12, it is not unusual, when the etching process is close to its end, for the etchant to leak through perforations in the wafer (such as 15) to attack the front side (see 16). There is, therefore, a need in the bulk wet etching art to be able to ensure not only etching uniformity but also leakage protection.
A number of methods and apparatuses have been invented to implement wet etching of silicon wafers, including those that incorporate various etch stop mechanisms, but little attention has been paid to the protection of wafers in case of leakage. Two typical techniques to prevent wafers from being attacked by etchant leakage are leakage detection and front side coating with etch resistant materials.
Leakage detection. This method uses electrodes or sensors to detect the presence of any alkaline etchant on the wafer front side. Once a leak is detected, the etching process is immediately aborted. However, in practice, it is found that leaks start from tiny pin holes and gradually spread over the wafer front side through capillary action and surface tension forces. It is often too late by the time a sufficient amount of leakage has occurred to be detected. This has resulted in the loss/scrapping of a number of wafers in production lines. In addition, aborting the etching process each time a leakage detector is triggered can bring operational difficulties and risks of wafer breakage.
Front side coating. In this approach, the wafer front side is coated with material that is resistant to the etchant. After the etching process, this coating is removed using an appropriate solvent. Typical coating materials are wax-based but some wax-based materials have properties that are incompatible with device processing or with the devices themselves and are therefore not permitted in a clean room. For example, methyl benzene, the solvent for removal of black wax, may not be used in most countries because it is hazardous to human health. Additional disadvantages reside in their cumbersome coating and removal processing. Furthermore, the coverage provided by the wax coating tends to be inadequate when the wafer front side is non-planar, containing stepped layers of different heights. In these cases, the leaked etchant can still spread out and attack the devices covered by the wax coating through undercutting.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 5,879,572, Folsom et al. show a bulk wet etch method while U.S. Pat. No. 5,338,416 (Mlcak et al.) shows a related etch process. Rolfson, in U.S. Pat. No. 6,025,278, shows a bulk wet etch process and apparatus in which the presence of etchant on the front side of the wafer is monitored and used to terminate etching once etch-through of via holes through the wafer has been achieved. By definition, no steps can be taken to prevent etch-through (as is done in the present invention). We note the following significant differences between this reference and the present invention:
As noted above, Rolfson is for end-point detection only and provides no protection to devices in the event of leakage. In the present invention the inert liquid (DI water) will dilute and remove any etchant leak so that devices are protected during the brief period between leakage detection and termination of etching.
In Rolfson, the inert and etchant liquids will exchange and/or mix at the end of wafer etching, whereas in the present invention the two liquids are made flow on the wafer surface for etching uniformity and leakage protection.
In Rolfson, the two liquids may be stagnant whereas in the present invention the etchant can be re-used since the inert liquid does not enter the etchant in case of leakage.
In Rolfson, neither liquid can be reused since they become mixed, thereby changing their properties.
In the present invention, liquids in the two chambers have independent flow rates and temperatures and maintain the same pressure on opposite sides of a wafer. This is not true for the Rolfson invention.